Rear End Fluid replacement

[JimBobC4]

I'm now to the rear portion of my 1995 6 speed 'vette. Next on my list this weekend is draining and refilling the rearend housing.

Again, I went to the GM dealer and was met with a strange look and after several minutes was handed a bottle of Fluid with a little bottle of friction modifier. However, the guy at the counter was really unsure of himself so I left it on the counter and now I'm asking you guy's.

I've read about pro's and cons of using synthetics, putting in 2 bottles of friction modifier and many other variations on this a few other sites.

Ok. What works ? Was the fluid in the housing synthetic originally ?

 

[LT4man]

OEM diff fluid was not synthetic. On the three (3) C4s which I purchased new, I changed the diff fluid over to synthetic as soon as I took delivery of the car.

GM says to add one container of additive. I have been told to always add two (2). I always do. The containers are only four (4) ounces. I guess better to be safe than sorry.

The correct weight oil is 75W/90.

I use a small pump driven off my 3/8" electric drill to remove the old fluid. Only takes about 10-15 minutes. Put in the two containers of additive. Put in the diff fluid until it just comes back out the fill hole. Wrap some teflon tape on the fill plug, reinstall plug and you will be good to go!

SAVE the gosh darn

 

[JimBobC4]

How do you get the additives into the housing ? Whats the secret there?
Just pour it in the little hole? :bash do a slurp and squirt with a new baster ?

Why did you go immediately to synthetics ?

I guess as far as lubricants go, that is the only one that isn't synthetic on the LT1. That is what had me curious about the use of the synthetics on the rear gears.

I've always used synthetic redlucaslinemysteryoil :eyerole in all the differentials of my past hobby cars, yet GM sends out a 50k car with petroleum based differential oil ?




thanks for the help !

 

[Edmond]

Synthetics just offer better all around protection. They have higher tolerances to heat and heat will destroy metal.

 

[andrewc4]

I went to Home Depot and bought a small black oil can that has a "trigger pump" and about a four inch long thin nozzle that easily fits in the fill hole. I marked it DIFF FLUID and I only use for refillling the rear end.

 

[LT4man]

How do you get the additives into the housing ?

I bought a quart plastic bottle with a hand pump on top from Auto Zone. Poured the additive into the bottle along with the diff fluid. Sure is easy to just pump the oil into the diff!

Why did you go immediately to synthetics ?

I have been using synthetics since 1975. As Edmond said in post #4 above, FAR superior to any dino-based oils.

I guess as far as lubricants go, that is the only one that isn't synthetic on the LT1.

Engine oil is the ONLY synthetic on LT1/4s.

yet GM sends out a 50k car with petroleum based differential oil ?

I have no issue with that. Helps to keep the cost of the Vette down.

SAVE THE

 

[Black Bart]

When I changed my diff. I drilled a hole and tapped it for a 1/8" pipe plug. I installed Mobil syn fluid no additive needed.
Conventional gear oil needs the friction modifier so the clutch pack don't slip.
Since it makes the oil less slick you should use what is recommended adding more will make the oil less slick and accelerate wear on the gears and bearings.
People take it upon themselves to add more but their is a very good reason why the manufacture recommends a certain amount.
I bought a cheap pump at Auto zone and two bottles of the syn gear oil and filled it up. Easy job.

 

[pgtr]

When I changed my diff. I drilled a hole and tapped it for a 1/8" pipe plug.

Did you drill & tap the 1/8" NPT in the bottom of your differential case while installed on the car with gears/oil in it?

And just to verify - you plugged it w/ a small 1/8" plug threaded for NPT? Not 1/4" or 1/2" ...

Thanks

 

[Black Bart]

Did you drill & tap the 1/8" NPT in the bottom of your differential case while installed on the car with gears/oil in it?

And just to verify - you plugged it w/ a small 1/8" plug threaded for NPT? Not 1/4" or 1/2" ...

Thanks

Yes 1/8" is plenty big enough for a drain hole.
I looked at a diagram of the rearend and I drilled off side of the ring gear.
I drilled with the oil in it with the fill plug removed.
After tapping I installed the drain plug and pumped a quart of 10w motor oil then drained it again.
Then I put in another quart of oil and drained it.
That flushed any chips that tapping may have left inside.
I used a pump that I got at Auto zone place one hose in the bottle of gear oil and the other in the fill hole and pump it in.
I used Mobil syn gear oil no need to add anything my possi works perfectly with it.
While changing the rear I change the transmission at the same time while I have it up on jack stands..
The fill plug on the tranny is hard to remove if it has not been out before so before you drain make sure you can get the fill plug removed.

 

[pgtr]

Thanks!

At some point would you care to post picture(s) or details on where you positioned the drain hole you drilled?

I second your advice on always loosening the fill plug first on any car before draining.

 

[JimBobC4]

Thanks!

At some point would you care to post picture(s) or details on where you positioned the drain hole you drilled?

I second your advice on always loosening the fill plug first on any car before draining.

Agreed. I would be interested in some pics as well. I know your car makes some huge power numbers so it must not create any structural integrity issues.

 

[JayThomas]

Agreed. I would be interested in some pics as well. I know your car makes some huge power numbers so it must not create any structural integrity issues.

Yikes!!! Drilled the housing while installed? How can you be sure you got all of the metal bits out?

 

[John Robinson]

When I changed the fluid in my 93 rear end I went to Harbor Freight and bought a big suction pump. (black in color) added about 2 feet of plastic hose to the end so I could get into the fill hole to suck out the fluid. As LT4Man said I also added two bottles of the positraction lube and then topped it off with syn. gear oil. To get the fluids back into the diff I used the over size baster to pull the fluid out of the bottle and then just squirted it into the fill hole. I think it only took me about less than 20 min. start to finish with a liquid brake. Since I only plan to change this every 100K I would not personnally go to the trouble of drilling and tapping a drain hole. After all when you drain the engine you do not get all the oil out of it and so why worry about what little there is left in the diff if you suck out the oil.

 

[Black Bart]

When I changed the fluid in my 93 rear end I went to Harbor Freight and bought a big suction pump. (black in color) added about 2 feet of plastic hose to the end so I could get into the fill hole to suck out the fluid. As LT4Man said I also added two bottles of the positraction lube and then topped it off with syn. gear oil. To get the fluids back into the diff I used the over size baster to pull the fluid out of the bottle and then just squirted it into the fill hole. I think it only took me about less than 20 min. start to finish with a liquid brake. Since I only plan to change this every 100K I would not personnally go to the trouble of drilling and tapping a drain hole. After all when you drain the engine you do not get all the oil out of it and so why worry about what little there is left in the diff if you suck out the oil.

I have not been on this site in a few years but got an e mail about this post.
Small metal particles will settle to the bottom and it is not likely that you will get them removed with a suction hose.

A stock engine will not need drained often I think GM recommends 30K but I change every 10K last time on the dyno it made 1178HP not what you would want to run 100,000 miles without service.

2 years ago I had back surgery and since then have only drove my vette twice and then only a few miles.

I will try to remember to get a photo if I every put it up on jack stands.
You can find diagrams of the differential just drill off to the side of the ring gear.

Drill with the oil in it and no fillings will get in the housing but I would flush it anyway to be sure.
I drilled mine 6 or 8 years ago, Since then I have drained it a few times sure is a nice up grade.

 

[toptechx6]

Agreed. I would be interested in some pics as well. I know your car makes some huge power numbers so it must not create any structural integrity issues.

See this link to a thread on the "other" forum then scroll down to my post #4, note that the exhaust is removed in the photos but the drain can be installed without removing it.

2 rear end questions - Corvette Forum

 

[Marv02]

You can use a pump to add the fuild or just buy a long hose stuff one end in the fill hose bring the pther out the whell well put a funnle on this end just pour the oil in untill it starts comming out the fill hole.

For the addtive put some oil in the diff stop pour the addtive inb the oil bottle them put the rest in the diff.

 

[john6277]

Rear end fluid replacement 1995 C4

Just finished this job. Reading forum messages helped, but here are my experiences.
I set up 2X8 single ramps on the drivers side and 4 layers of the same on the right side. Now I could fit under the auto and have the fluid level tilted. Inspection/filler plug may have had a allen wrench hole, but this center hole was worn beyond recognition. My medium sized plumbers pipe wrench got a good grip on the outer lip of the bolt and it came out fine with adequate room to move wrench. Tried to buy the new GM plug but gave up on finding a source.
Inserted a 1/4 ID vacuum hose and fished it down to bottom of the left side. A few pumps with a turkey baster started the syphon flow for about 15 minutes. This captured all but 3-4 ozs of the 1.5 quart capacity. Using the same hose on the tapered bottle top I premixed and squeezed the Valvoline VV831 80-90 GL 5 lube and 4 ozs compatible TransX additive per Corvette manual. All is well and I'll do this again in a few years as I only put on 5-8 K miles per year.

 

[Hib Halverson]

There are a couple of different ways to get the old gear lube out of a C4 rear axle and both have been covered adequately. I prefer the drain plug route, so a number of years ago, I modified the rear axle in my 95 with a drain plug.

As for the lubricant–I use Red Line Heavy Shockproof Gear Lubricant exclusively and I have used it for nearly 20 years. I run it 36,000-40.000 miles then change it. While Red Line Heavy Shockproof has a friction modifier blended in I usually end up adding an additional half to 3/4s ounce of friction modifier.

Because of common practice by dealers and service shops along with a healthy dose of Internet rumor, many people automatically use a "additive" in Corvette rear ends.

These "additives" are all "friction modifiers" which were originally intended to solve customer complaints of noise or chattering coming from the rear end during slow speed turns. The noise or chatter comes from the friction between the plates of the clutch-type limited slip differential used in C4s. The limited slip needs friction to work properly, but this friction can also cause noise (a sort of squealing) or in more severe cases, chattering.

The problem with these additives comes when they are misused. Most service shops and dealers, because they don't want to take the time required to find the exact amount of additive necessary to solve the problem, often ad a full bottle as a matter of practice during a rear axle lubricant change. Doing it that way doesn't get you the best compromise between friction and reduced chatter or noise.

Remember–the limited slip in a C4 needs friction to work properly so when you use a friction modifying additive you are degrading limited slip action.

What I do is: I change the gear lubricant, then I road test the car. I drive for 10 minutes on the highway to bring the lube up to operating temperature. Then I find a large empty parking lot and drive in slow, tight circles, one in each direction, listening for rear end noise or feeling for chatter.

If the limited slip chatters, I go back to the shop and add one ounce of friction modifier. Road test again. If the chatter is still present, add a second ounce then road test, again. If there is still chatter, add a third ounce. After the third ounce if chatter persists, I tell people they need to have the rear axle overhauled.

In the case of squealing but no chatter, I am very careful with any friction modifier because first and foremost, I want my limited slip to work. If the squeal is loud, I'll add half or 3/4-ounce of friction modifier and road test. If after that, the squeal is still present but subdued, leave well-enough alone figuring I've got the best compromise between low noise and high limited slip performance.

Lastly as for the choice of lubricants: not everyone is going to want the expense of Red Line's Heavy Shockproof Gear lubricant also some people are put off by the gel-like consistency of its calcium-based EP additive package and it's pink color. There are other Red Line gear lubes which have a more conventional consistency and appearance. Additionally, Driven Racing Oil (the consumer lubricant blending operation owned by NASCAR team, Joe Gibbs Racing) makes an excellent 75W90 rear axle lubricant for limited slip axles. For those looking for lower cost, you can use a petroleum-based gear lubricant but make sure it meets GL5 and is rated as 75W90 or an 80W90.

As for friction modifiers, the GM limited slip additive works well with most gear lubes except the ester-based products such as Red Line. If you use Red Line gear lubes, I'd recommend Red Line's Friction Modifier as it's compatible with all Red Line's gear oil formulae.

 

[froggy47]

I know it's an old thread,

Has anyone found a good diff drain plug with magnetic insert? Bulk they cost maybe a buck to make, so I would prefer not to be overcharged too much (Vette tax).

Thanks.